For example, on various vehicles such as an electric vehicle (EV) which travels using an electric motor and a hybrid electric vehicle (HEV) which travels using an engine and an electric motor, a secondary battery such as a lithium ion charging battery or a nickel hydrogen charging battery is mounted as a source of power for the electric motor.
It is known that such a secondary battery is degraded gradually by repetition of charging and discharging and a storable capacity (such as a current capacity and a power capacity) decreases gradually. In an electric vehicle or the like using a secondary battery, a storable capacity is found by detecting the degree of degradation of the secondary battery and a distance over which the vehicle can travel using the secondary battery, the life of the secondary battery, or the like is calculated.
As one of indexes indicating the degree of degradation of the secondary battery, there is SOH (State of Health), which is a ratio of a current storable capacity to an initial storable capacity. It is known that the SOH has correlation to internal resistance of the secondary battery. If the internal resistance of the secondary battery is found, the SOH can be detected based on the internal resistance.
For example, a direct current signal that becomes constant in voltage and current, or an alternating current signal that becomes constant in voltage and current waveforms is applied to the secondary battery. The internal resistance of the secondary battery can be found based on a response thereof. An example of technique of measuring such internal resistance of the secondary battery is disclosed in Patent Literature 1 and so on.
In an assembled battery formed by combining a plurality of single cell batteries, a single cell battery that becomes a measured object (hereafter referred to simply as “measured battery”) is energized from the external. At this time, according to a method disclosed in Patent Literature 1, a choke coil is provided between the measured battery and other single cell batteries connected adjacent to the measured battery to suppress divided currents that flow through single cell batteries other than the measured battery. As a result, influence of other single cell batteries connected to the measured battery is excluded. A voltage of the measured battery before energization and a voltage of the measured battery at the time when a predetermined time has elapsed since the energization are measured. Internal resistance is calculated from a difference voltage between these voltages and the energization current from the external.